Yea i used spaghum moss in one of Thor's hides when i was having humidity problems. I keep my water dish halfway on the UTH and halfway on the cool side, and it seems to work well.

Now that I've thought about it, I think your problem lies in the foam board. Your area is so small now, that the lamp is probably drying up ALL humidity very fast. You really don't need the enclosure to be 1.5' x 1.5' as that is kinda overkill. Provide enough hides/driftwood/plants that it makes the snake feel secure in the full size of the tank. This will allow the humidity to build throughout the tank without it drying up under that 75watt lamp you have (which is way too much for a 1.5x1.5 area).

Edit: I'd take that foam board and fasten it to the outside of the tank on 3 sides (leaving the front side open for viewing). Theres a guide on these forums on how to do it I think. I know I saw it somewhere.